Slug is typically used to push mud in the drill string down therefore pipe will dry while pulling out of hole. Dry pipe while pulling out has some advantages as minimizing crew to expose to drilling mud when breaking a connection, reducing time to handle the drill string, etc. Sometimes, even you already pump slug but you still have wet pipe instead of dry pipe. You may wonder why the slug does not work very well.
The following reasons why the slug does not do its job are as follows:
• Slug volume is not enough to slug the pipe. Recommended volume is around 25- 40 bbl.
• You should chase slug by pumping mud at least surface volume from mud pump to a rotary table. Otherwise, you will not get desired slug volume in the drillstring because it is still left in the surface volume.
• Weight of slug is not sufficient. As a normal practice, the slug weight should have at least 2 ppg over your current mud weight.
• There is something inside the drill string so the slug could not push mud in the drill string down.
If you want to learn more about slug, please read the following articles: